Organizational Risk Factors for Aircrew Health: A Systematic Review of Observational Studies

Addressing the field of health and safety at work, the primary objective of the present systematic review was to analyze the organizational risk factors for aircrew health according to professional category (flight attendants and pilots/co-pilots) and their consequences. The secondary objective was to identify the countries in which studies were carried out, focusing on the quality of content of the publications. The Medline/Pubmed, Cochrane, Web of Science, and Scopus databases were searched for eligible studies according to PRISMA statements. The risk of bias and the methodological quality of the studies were assessed using the Newcastle-Ottawa scale and Loney tools. Of the 3230 abstracts of articles screened, 36 studies met the inclusion criteria. Most of the research conducted on risk factors for the work organization of aircrew was carried out in the United States and the European Union and had moderate or low-quality methodology and evidence. However, the findings are homogeneous and allow the most prevalent organizational risk factors for the health of aircrew to be determined, namely, high work demand, long hours, and night work. Consequently, the most pervasive health problems were sleep disturbances, mental health disorders, musculoskeletal disorders, and fatigue. Thus, the regulation of the aircrew profession must prioritize measures that minimize these risk factors to promote better health and sleep for these professionals and, consequently, to provide excellent safety for workers and passengers.


Introduction
In Brazil, the demand for studies on the working conditions of aircrew and risks they are exposed to during work shifts has been growing in recent years. These demands stem from associations and organizations representing aviation professionals, as well as from airlines and government seeking to determine the impact on the social security system. Hence, these three dimensions intrinsically involve the regulatory framework governing the aviation industry and, consequently, related public policies.
In recent years, the civil aviation sector has grown significantly. According to the Brazilian Civil Aviation Agency (ANAC), in 2019, a record number of passengers (119.4 million) were carried on commercial flights in the country [1]. However, this high work demand, together with changes in flight schedules, stress, and psychological pressure, can cause fatigue, directly affecting the health of aircrew (Melo and Silvany Neto, 2012). This situation has given rise to a number of issues, such as instability, intense work pace, long working hours, irregular working hours, reduced pay, and loss of control over job activities [2,3].
Previous studies by Goode [4], Powell et al. [5], Marqueze et al. [6], Goffeng et al. [7], and Pellegrino and Marqueze [8] have highlighted that organizational aspects, such as long working hours, work demands, and schedules, numbered among the main risk factors for aircrew health. However, no study was found in the literature that summarized the 2 of 29 findings on risk factors for these professionals. A study that synthesizes the main risk factors and their consequences for the aircrew's health will be important to be able to think about health polices for these workers, as well as to synthesize what we already know about the subject and what still needs to be researched. Therefore, the primary objective of the present systematic review was to analyze the organizational risk factors for the health of aircrew according to professional category (flight attendants and pilots/co-pilots), as well as their consequences. The secondary objective was to identify the countries in which studies were carried out, focusing on the quality of content of publications.

Materials and Methods
A systematic review was performed according to the guidelines of the PRISMA checklist (Preferred Reporting Items for Systematic Reviews and Meta-analyses) [9] available in the Supplementary Materials (Table S1) and registered on PROSPERO under number 240012. Given this was a review of the literature, approval by the Ethics Committee was not required.
The PICO strategy for non-clinical research was used to devise the research question (Table 1): what are the organizational risk factors for aircrew health? Pubmed, Cochrane, Web of Science, and Scopus databases were systematically searched from February to May 2021 to identify studies related to the research question. Keywords were defined according to medical subject headings (MeSH) and are presented in Table 2. The references of each study retrieved were manually investigated to identify additional eligible studies. Table 2. Search terms on electronic databases.
Outcomes "Occupational Risks" or "Risk Factors" or "Work hours" or "Workload" or "Shift Work Schedule" or "Night Shift Work" Population "Aircrew" or "Flight attendants" or "Air Pilots" or "Aviators" or "Co-Pilot" Limit "Full-text available" and "Peer-reviewed papers" Potentially relevant studies were selected by two independent reviewers based on the following inclusion criteria: (1) studies in Portuguese, English, and Spanish only; (2) full studies; (3) observational (cross-sectional, cohort, case-control) and intervention studies only; (4) articles published after 1990; (5) studies involving men and women from any age group; (6) studies whose objective was to assess occupational risks, analyze their impact on aircrew health, and describe ways of mitigating these factors; (7) peer-reviewed studies; (8) studies on commercial aviation only. Exclusion criteria included clinical trials, ecological studies, reviews, qualitative studies, studies of retired aircrew, studies with objectives different from those of the present review (off-topic), abstracts, technical reports, oral communications, letters to the editors, and studies of flight simulation.
Article selection was performed independently, in a double-blind fashion, by two reviewers who screened all titles and abstracts. Subsequently, both reviewers read the full papers that met the inclusion criteria. In the case of disagreement regarding the eligibility of studies, a third researcher was consulted to reach a consensus.
The researchers created an electronic synthesis form to extract data from the papers reviewed. Data extraction covered the following variables: authors, publication year, study

Study Characteristics
The place of origin of the publications reviewed is shown in Figure 2. Most of the studies were carried out in the United States (22.2%), followed by Germany (11.1%) and Sweden (11.1%). Brazilian studies accounted for 8.3% of the publications selected.

Study Characteristics
The place of origin of the publications reviewed is shown in Figure 2. Most of the studies were carried out in the United States (22.2%), followed by Germany (11.1%) and Sweden (11.1%). Brazilian studies accounted for 8.3% of the publications selected.
The analysis of the results found in the studies involving flight attendants revealed that the main risk factors for work organization were high work demands (physical and psychological), harassment, and low job experience. The main consequences of these risk factors included sleep problems, cardiovascular diseases, mental health disorders, musculoskeletal disorders, injuries, and low work capacity (Table 3). The studies involving flight attendants were conducted between 1998 and 2019, 25% of which were published in 2019 ( Table 3). The studies involving pilots were carried out between 1997 and 2020, with 30.4% published in the last five years (Table 4). Lastly, studies involving both flight attendants and pilots were conducted over the period 2002-2021, 70% in the last five years (Table 5). Overall, 36.1% of the studies reviewed were published during the period spanning from 2017 to 2021.
The analysis of the results found in the studies involving flight attendants revealed that the main risk factors for work organization were high work demands (physical and psychological), harassment, and low job experience. The main consequences of these risk factors included sleep problems, cardiovascular diseases, mental health disorders, musculoskeletal disorders, injuries, and low work capacity (Table 3).
Lowden and Åkerstedt [12], in a prospective cohort study of cabin crew over a nineday period, found that shorter rest time between work shifts (flights with quick layovers) led to decreased sleep time and efficiency. Sleep was not restorative, making it difficult to wake up and, consequently, increasing drowsiness during wakefulness. In the retrospective cohort study of Zeeb et al. [13], involving male cockpit crew members only, it was found that those who started working at age 30 or older had more than double the risk of cardiovascular mortality compared to professionals who entered the profession earlier.
No risk gradient was observed for duration of employment (Table 3).
Ballard et al. [14], in a cross-sectional study of flight attendants, found that low job satisfaction and sexual harassment by passengers were risk factors for psychological distress. Lee et al. [15], also in a cross-sectional study involving flight attendants, identified both physical and mental work demands as risk factors for musculoskeletal disorders. Furthermore, in a cross-sectional study conducted by Castro et al. [16] investigating cabin crew members, the authors lack of humidity/moisture in the cabin, longer working experience, and being female were also risk factors for fatigue, poor sleep quality, and body aches. These factors, as well as their outcomes, can contribute to decreased work capacity (Table 3).
In a cross-sectional study by Hu et al. [17] of cabin crew, purser, and cabin manager, the authors confirmed that exhaustion from work, emotional distress, and insomnia were risk factors for decreased work ability. However, in a cross-sectional study of flight attendants conducted by Widyanti and Firdaus [18], a moderate mental workload was noted, regardless of flight duration. Lastly, Agampodi et al. [19], in a cross-sectional study of flight attendants, found that females (the predominant gender in this role), low weight The studies involving flight attendants were conducted between 1998 and 2019, 25% of which were published in 2019 ( Table 3). The studies involving pilots were carried out between 1997 and 2020, with 30.4% published in the last five years (Table 4). Lastly, studies involving both flight attendants and pilots were conducted over the period 2002-2021, 70% in the last five years (Table 5). Overall, 36.1% of the studies reviewed were published during the period spanning from 2017 to 2021.       To demonstrate an empirical relationship between pilot schedules and aviation accidents.
The proportion of accidents associated with pilots having longer duty periods is higher than the proportion of longer duty periods for all pilots. For 10-12 h of duty time, the proportion of accident pilots with this length of duty period is 1.66 times as large as for all pilots. For pilots with 13 or more hours of duty, the proportion of accident pilot duty periods is 5.62.
For the analysis, one month of flight activity data was compared with 20 years of accident data arguing that the distributions of these data sets should be the same if length of duty has no impact on accidents. However, one may wish to be cautious of the empirical findings based on this assumption. If these distributions were not expected to be the same in the first place, then the findings are problematic. With this potential weakness in mind, the findings do strongly suggest that there is an increased probability of an accident as duty time increases, and therefore more stringent limitations on pilot duty time may be appropriate. The results show that total sleep times did not differ between morning and evening flight crews, despite the fact that morning crews had to arise earlier before the flight and evening crews went to bed significantly later during the layover. Apparently, the pilots compensated for the effects of the flight by going to bed earlier, and sleeping longer in the morning, respectively. The total sleep time over the days varied, however, with shorter periods of sleep the night before the outward-bound flight and the greatest amount of sleep during the night after returning home, for both morning and evening flights. There was no difference between the morning and evening flights with regard to total sleep time, bedtimes, or rising times over the course of the study. Actigraphy showed no differences between morning and evening flights, which seems to indicate that the need for recovery was not affected by working during the daytime or night. Evening flights involve higher levels of sleepiness than morning flights, presumably because of the close proximity in time to the circadian trough of alertness.
None declared by the authors. To investigate how length of duty, number of sectors, time of day, and departure airport affect fatigue levels in short-haul operations.
The most important influences on fatigue were the number of sectors and duty length. These were associated with fatigue in a linear fashion. Time of day had a weaker influence, with lower levels at midday and increased fatigue later in the day. Fatigue was also higher during duties originating from an airport where pilots needed to position the night before and spend the night in a hotel. Data from the study enabled the quantification of fatigue at this critical phase of flight in duties lasting between 2 and 12 h and finishing between 08:00 and 24:00.
There are a number of methodological issues relevant to the interpretation of the results. Firstly, fatigue was measured using subjective pilot ratings, rather than using objective indices, such as re-action time tasks. However, the Samn-Perelli scores have been shown to follow similar trends to objective measures throughout a duty period (8). Secondly, all the data were collected on domestic sectors of a similar length with no overnight duties and no time zone changes, so they may not be directly applicable to different operations. It is also likely that this lack of overnight duties attenuated the time of day effect as highest levels of fatigue tend to occur in the early morning.
Cohort prospective An amount of 19 male pilots (10 Captains-mean (±SD) age of 53.9 (±1.9) yrs, 9 First Officers-mean age of 45.6 (±6.4) yrs) To examine the impact of layover length on the sleep, subjective fatigue levels, and capacity to sustain attention of long-haul pilots.
The pilots in this study obtained substantially more sleep/d during their layover than they did during their pre-trip days off. Although the interaction was not statistically significant, it seems apparent that the effect of the layover on sleep was mainly due to the group of pilots that had a short layover. Indeed, these pilots obtained a mean of 8.6 h of sleep/d during the layover, compared to 7.2 h of sleep/d during pre-trip days off. As with most long-haul flights, the outbound flight in this study involved a very long duty day (~16 h). Thus, not surprisingly, pilots had substantially higher fatigue levels at the end of the flight than at the start, as indicated by their subjective ratings and their capacity to sustain attention. The need to recover from the sleep debt and fatigue accumulated during the outbound flight could account, at least in part, for the fact that pilots obtained more sleep during layover days than they obtained on days off at home prior to the trip. The results of this study indicate that a short layover during a long-haul trip does not substantially disrupt pilots' sleep, but it may result in elevated levels of fatigue during and after the trip. If short layovers are used, pilots should have a minimum of 4 d off to recover prior to their next long-haul trip.
First, the number of participants was limited by time and cost restraints. If this resulted in the study being underpowered, then the likelihood of committing a type II error would have been elevated. Second, participants were not randomly allocated to the two layover groups by the authors, but instead they were assigned to a long-haul trip that contained either a short or a long layover in an optimization process conducted by the employer airline. If there was some bias in the assignment of trips-a possibility that exists given that participants had an opportunity to bid for the trips that they preferred-then any resultant differences between the two groups may have affected the results. Third, there was no marker of circadian phase (i.e., body clock time), such as dim-light melatonin onset or minimum of the core body temperature rhythm. Therefore, it was not possible to examine whether differences between the two groups of pilots in terms of their subjective fatigue levels and their capacity to sustain attention were influenced by differences in the degree to which the timing of their body clocks were altered during the short and long layovers.        The findings suggest that a higher self-reported work-load is associated with slower reaction time and higher ratings of fatigue. There were very weak, but significant, correlations between mean pilot workload as measured by NASA-TLX and PVT performance, Samn-Perelli ratings, sleep duration, number of sectors, and flight duration. In addition, each workload scale was significantly correlated with PVT performance and Samn-Perelli fatigue ratings. Pilots experienced more PVT lapses, and their response speed was slower when workload was rated higher. Albeit still weak, the highest correlations of the study were between PVT performance and work-load; workload was rated higher when fatigue was also rated as higher.
Pilots reported being frustrated, experiencing time pressure, and making more effort on shorter flights. The stepwise regression showed that Samn-Perelli fatigue, the number of sectors and PVT lapses were significant predictors of workload, although the effect was very small.
Although significant, the correlations in the present study were very weak and the variance explained by the predictors in the regression model was very small. In addition, the study shows relationships between these factors but does not indicate causality. An additional limitation is that the TLX ratings referred to the flight up to the top-of-descent and it didn't capture landing, which is the most stressful phase of flight. Finally, the flight sectors studied were relatively short due to the short-haul nature of the study, which may explain why flight duration was not a significant predictor in the regression model.
Lowden and Åkerstedt [12], in a prospective cohort study of cabin crew over a nine-day period, found that shorter rest time between work shifts (flights with quick layovers) led to decreased sleep time and efficiency. Sleep was not restorative, making it difficult to wake up and, consequently, increasing drowsiness during wakefulness. In the retrospective cohort study of Zeeb et al. [13], involving male cockpit crew members only, it was found that those who started working at age 30 or older had more than double the risk of cardiovascular mortality compared to professionals who entered the profession earlier. No risk gradient was observed for duration of employment (Table 3).
Ballard et al. [14], in a cross-sectional study of flight attendants, found that low job satisfaction and sexual harassment by passengers were risk factors for psychological distress. Lee et al. [15], also in a cross-sectional study involving flight attendants, identified both physical and mental work demands as risk factors for musculoskeletal disorders. Furthermore, in a cross-sectional study conducted by Castro et al. [16] investigating cabin crew members, the authors lack of humidity/moisture in the cabin, longer working experience, and being female were also risk factors for fatigue, poor sleep quality, and body aches. These factors, as well as their outcomes, can contribute to decreased work capacity (Table 3).
In a cross-sectional study by Hu et al. [17] of cabin crew, purser, and cabin manager, the authors confirmed that exhaustion from work, emotional distress, and insomnia were risk factors for decreased work ability. However, in a cross-sectional study of flight attendants conducted by Widyanti and Firdaus [18], a moderate mental workload was noted, regardless of flight duration. Lastly, Agampodi et al. [19], in a cross-sectional study of flight attendants, found that females (the predominant gender in this role), low weight (under 56 kg), and shorter work experience (<7 years) were risk factors for injuries on board ( Table 3).
The most investigated topics among pilots were fatigue, drowsiness and alertness, which are frequent complaints among these professionals. In chronological order, the first study [20] with a prospective cohort design showed that long-haul flights, night flights, and short rest time between flights were associated with fatigue. In a cross-sectional study, Gander et al. [21] demonstrated that night flights, as well as short sleep duration, increased fatigue and decreased alertness levels led to poor eating habits, headaches, and backache among pilots. According to a cross-sectional study by Eriksen and Åkerstedt [22], night flights involve higher levels of sleepiness than morning flights for pilots (Table 4).
In a three-month prospective cohort study, Powell et al. [5] reported an association of long working hours and greater number of daily flights with a higher level of fatigue. Similarly, Roach et al. [23], in a prospective cohort, found that long working hours and sleep restriction increased fatigue and decreased level of attention among pilots. In a crosssectional study carried out by Gander et al. [24], long working hours, with consequent sleep restriction on trans-meridional flights (crossing 7-9 time zones westwards), were also shown to affect pilots' alertness level (Table 4).
In a cross-sectional study, Reis et al. [25] found that short-and medium-duration flights were factors associated with mental and physical fatigue among pilots. The authors explained this result as due to a large number of takeoffs and landings in shorter flights, events that demand the greatest attention and action of pilots. In a cross-sectional study by Gander et al. [26], the authors found that at top of descent (TOD-computed transition from cruise phase of flight to descent phase) and flights that lasted between 6:00 and 9:59 h led to most fatigue and impairment of pilots' reaction time. Vejvoda et al. [27], in a cross-sectional study, also identified extended wakefulness and short-haul flights ending late at night as risk factors for fatigue (Table 4).
In a cross-sectional study, Van Drongelen et al. [28] showed that higher age, being an evening type, disturbance of work and personal-life balance, more need for recovery from work, lower perceived health, less physical activity, and moderate alcohol consumption were risk factors for fatigue. Lastly, Arsintescu et al. [29], also in a cross-sectional study, observed that higher workload, short sleep, greater number of daily flights, and short flights were factors associated with fatigue and impairment of pilots' reaction time (Table 4).
According to Goode [4], pilots' long working hours are risk factors for work accidents. Working days that last approximately 10-12 h increase the risk of accidents by 66%, and days of 13 h or more were associated with a 4.62 times greater risk. Another major problem among pilots is unintentional sleep during flights. In a cross-sectional study by Marqueze et al. [6], the prevalence of unintentional sleep was almost 60% among the pilots assessed. The authors found that longer monthly flying hours, frequent technical delays, greater need for recovery after work, work ability below optimal, insufficient sleep, and excessive sleepiness during waking hours were risks factors for unintentional sleep (Table 4).
Fatigue resulting from these risk factors can lead to other problems among pilots, as demonstrated by O'Hagan et al. [30]. In a cross-sectional study conducted by these authors, high work-related fatigue, frequent sleep disturbances, and unintentional naps during work were associated with depression and anxiety. Other factors were also associated with mental problems in this professional category. Feijó et al. [31], for instance, in a crosssectional study, identified high work demand and sedentary lifestyle as risk factors for common mental disorders among pilots. The cross-sectional study by Demerouti et al. [32], also established that high demand and effort at work are risk factors for burnout in the same population. In addition to these factors, Wu et al. [33], in a cross-sectional study, found that the use of sleeping pills and harassment at work were associated with depression among pilots (Table 4).
Another frequent complaint among pilots is musculoskeletal disorder. In a crosssectional study, Lawson et al. [34] observed that long flights are predictive factors for neck pain. Moreover, in a cross-sectional study, Runeson-Broberg et al. [35] identified having a high work demand, being female, having low social support, and suffering from psychosocial stress as risk factors for musculoskeletal symptoms. Albermann et al. [36], in a cross-sectional study, identified a high prevalence of chronic low back pain among pilots (82.7%) and a significant association with low back pain in those who had a total flight time of more than 600 h in the previous 12 months (Table 4).
Another problem studied that affects pilots was metabolic disorders. In a prospective cohort study, Zhao et al. [37] showed that pilots aged 31-50 years had a high prevalence of dyslipidemia compared to the general male population in China. De Souza Palmeira and Marqueze [38] observed a high prevalence of overweight among pilots (53.7%). Longer night work hours, difficulty relaxing after work, short sleep duration, chronic diseases, and a sedentary lifestyle were associated with excess weight. These problems, according to Bhat et al. [39], can lead to the development of other diseases. The authors found a positive correlation between overweight and hypertension among pilots, especially in individuals aged 26-35 years (Table 4).
Based on the results obtained in the studies of pilots only, the main organizational risk factors for the health of aircrew were long working hours, night work, little rest time between workdays, sleep restriction, and long waking hours due to working hours, high number of daily flights, low social support, organizational stress, frequent technical delays, lack of work and personal life balance, high work demand, and long-haul flights. The main health outcomes of these factors were physical and mental fatigue, lower alertness, poor eating habits, headaches, muscle pain, injuries, mental disorders, metabolic problems, unintentional sleep, and excessive sleepiness (Table 4).
In studies of flight attendants and pilots, other health issues were investigated besides fatigue. Ballard et al. [40], in a retrospective cohort study evaluating the association of causes of death (malignant neoplasms, non-cancerous, non-injurious, and external causes) with job position and gender, found that males with longer working hours were at higher risk of leukemia, while flight attendants (female) were at higher risk of suicide. In the cross-sectional study of Omholt et al. [41], flight attendants had a higher prevalence of musculoskeletal symptoms, as well as psychological, gastrointestinal, and allergy complaints, compared to pilots. In total, 20% reported high stress levels, which were associated with these symptoms and complaints. Regarding restless legs syndrome (RLS), Düz et al. [42], in a cross-sectional study, found that the prevalence of RLS among pilots and flight attendants was similar to that of the general population, and that flying at high altitudes was not a risk factor for the syndrome ( Table 5).
The studies by Goffeng et al. [43] and Åkerstedt et al. [44] evaluated both pilots and flight attendants. The first of these investigations, a seven-day prospective cohort study confirmed higher number of consecutive workdays (≥days), short sleep, less rest time between flights, and high workload as risk factors for greater cardiovascular effort among pilots and flight attendants. More specifically, Åkerstedt et al. [44], in a 14-day prospective cohort of flight attendants, found that long working hours, night work, short sleep, and very early flights were risk factors for fatigue in this group (Table 5).
In summary, these studies found that the main work organization risk factors for these professionals were longer work experience, consecutive working days, sleep restriction due to working hours, high work demand, long working hours, and very early flights. The main outcomes of these factors were the development of cardiovascular diseases, mental disorders, and fatigue (Table 5).
Notably, none of the studies included in this review researched ways of mitigating the risk factors investigated. The study authors only suggested measures to minimize health risks, including a reduction of consecutive workdays, especially night work, longer rest time between shifts, particularly after long-haul flights crossing time zones, provision of a suitable rest area on planes, optimizing the work by support team to shorten flight turnaround times, avoiding work schedules with very early starts, having greater social support at work, and prioritizing rest days on weekends.

Quality Assessment
The results of the quality assessment of the study designs according to the Newcastle-Ottawa or Loney criteria are given in Table 6. Out of the 36 articles reviewed, eight (22.2%) involved flight attendants only, 23 (63.9%) pilots only, and five evaluated both flight attendants and pilots (13.9%). Regarding the eight studies of flight attendants only, two (25%) were cohort studies, and six were cross-sectional (75%). Only one of the crosssectional studies was rated with 7 points, i.e., of 'high methodological quality'. Two studies were rated with five points, one with four points, and two studies were attributed a rating of 3 points, indicating that studies were predominantly of low methodological quality.
Regarding the studies of pilots only (n = 23), most were cross-sectional (n = 19, 82.6%). Of this total, four were rated with 6 points, indicating better quality. Eleven studies had a rating of 5, and two were attributed four points, indicating moderate quality. The remaining two studies scored only 3 points, i.e., were of low methodological quality. Only four publications were prospective cohort studies (17.4%), while none had a high level of evidence. Three of the studies had a moderate level of evidence, comprising two with 5 points and one with 4 points, while one had a low level of evidence (3 points) ( Table 6).  The impact of being at a high altitude on RLS is controversial. The RLS frequency in aircrew was 6.7%, which is similar to that of the normal population. It is considered that this similarity is due to modern technology which regulates and adjusts oxygen saturation and air pressure inside the aircraft throughout the flight. We can conclude flying at high altitude was not a risk factor for RLS.    For the three studies involving both flight attendants and pilots, most were cohort studies (two prospective and one retrospective-60%). Of these studies, two had moderate evidence (one with four points and another with five points), and only one study had low evidence. Of the two cross-sectional studies (40%), one scored five points, indicating moderate quality, and the other scored three points, indicating low quality (Table 6).

Discussion
The most prevalent organizational health risk factors faced by flight attendants and pilots were long hours, high work demand, and night work. These same risk factors are often found in other work activities also involving irregular hours and night shifts. An estimated 30% of the adult population are engaged in shiftwork, including night work [46]. According to the Working Time Society consensus statements, shift and night work are risk factors for workers' health, as circadian desynchronization, sleep restriction, and social misalignment can cause health problems (directly or indirectly) as a result of exposure tonight work [47]. Shift and night work lead to acute and chronic disturbances in sleep and alertness, increased risk of fatigue-related accidents and incidents, cardiovascular problems, metabolic disorders, musculoskeletal disorders, increased risk of developing cancer (prostate, colorectal and breast), unhealthy behavioral changes, such as those related to timing of meals and low physical activity, and gastrointestinal and digestive disorders [46][47][48][49]. Moreover, mental health problems are common in shift workers. According to Boivin et al. [46], these conditions develop due to long-term exposure to work schedules, leading to the disruption of the sleep/wake cycle and circadian desynchronization, significant risk factors for mental health.
In a systematic analysis carried out jointly by the World Health Organization (WHO) and the International Labor Organization (ILO), individuals who work long hours have a higher risk of developing ischemic heart disease and stroke [50]. Other systematic reviews have also revealed that long working hours are associated with burnout syndrome [51], decline cognitive capacity [52], depression [53,54], increased alcohol consumption [55], mental health disorders [56], and adverse pregnancy outcomes (preterm birth, chance of miscarriage, low birth weight) [57]. High work demands related to working hours and shift and night-work are also associated with problems such as burnout and anxiety [58], musculoskeletal disorders [59], sleep disorders [60], emotional exhaustion [61], and sedentary lifestyle [62].
In the case of aircrew, in addition to hours spent in flight, commuting time, especially for professionals living far from their designated place of work or staying in hotels, should be taken into account. Hotels located far from airports will mean longer commutes to work. Furthermore, the authors note that, even when at work base, most airports are far from cities and require a long commuting time. Moreover, it is necessary to account for the time needed to carry out briefings and administrative activities, as well as the time engaged at the end of the workday, which ends not when the engines are shut off but when aircrew are at home to rest. Technical delays are also a factor that can extend work hours of aircrew and should thus be taken in account [6].
As a result of these risk factors, the most prevalent health problems reported in the studies analyzed were related to sleep, mental health disorders, musculoskeletal disorders, impaired cognition and performance, and fatigue. Sleepiness and impaired cognition and performance are common among airline pilots [63][64][65]. In a study by Bostock and Steptoe [66] of airline pilots, based on an analysis of salivary cortisol, the authors found a decrease in wake-up response time on shifts that started either very early or very late, directly impacting sleepiness and impaired cognition and performance. According to Kecklund and Axelsson [67] and Fischer et al. [68], shift work and its deleterious effects on workers' sleep and concentration impair alertness levels and increase the risk of fatiguerelated accidents.
As reported by Wingelaar-Jagt et al. [69], fatigue is related to several factors, including sleep debt, long wakefulness, circadian desynchronization and high workload, has several deleterious effects on health, and decreases aircrew performance during flight. Problems associated with fatigue in aircrew include decreased alertness, mental health problems, excessive sleepiness, sleep disturbances, and increased cardiovascular effort [8,43,70,71]. Although regulations limit flight time, it is noteworthy that fatigue cannot be mitigated entirely and the best way to avoid it is to obtain adequate sleep at night for recovery [69].
Wingelaar-Jagt et al. [69] identified some factors in aviation that contribute to fatigue, namely, long-haul flights, mainly as a result of jet lag, night flights, and circadian rhythm disturbances; short-haul flights, because of the high work demand and sleep loss for having to wake up very early to work or sleep late due to late flights; international flights; problems with work schedules, such as many consecutive nights of work, lack of a suitable rest area on the aircraft, and flight schedules, especially those starting very early or ending late at night; high number of flights; and long working hours, while taking into account, in addition to flight time, the time dedicated to briefings, administrative activities, etc.
Regarding the quality of the studies analyzed in the present review, most were crosssectional (75%). In cross-sectional designs, a cause-and-effect relationship between risk factors and outcomes cannot be established, only indicating the existence of any associations between the variables studied [72]. However, these studies contribute significantly to the understanding of which organizational factors are associated with the health problems of aircrew, as well as helping identify the most frequent issues.
These data are important for designing and planning new studies, specifically longitudinal studies to elucidate the impact of work organization on the health of these workers and contribute to the management and care of these professionals. It is also essential to carry out dose-response studies to assess biological and behavioral factors that may impact individual susceptibility to the work schedules to which flight attendants and pilots are exposed, in addition to identifying mediating factors between risk factors and health outcomes. Social, economic, and legal aspects should be simultaneously considered and evaluated [45]. On the other hand, it is important to emphasize that most of the analyzed studies (86.1%) were classified as having moderate or low methodological and evidence quality, pointing to the need for more robust studies with fewer biases.
Finally, when considering the places where the studies were carried out, according to the Science and Engineering Indicators report [73], the United States and the European Union are the places that publish the most scientific studies in the worldwide, especially in the area of health. This same trend was also found in the present review, which analyzed studies conducted mainly in the United States, followed by Germany and Sweden.
This review contributes to the growing body of evidence suggesting that the work of aircrew has relevant risk factors which need to be better understood. Further studies, especially those with longitudinal designs, should be carried out to provide more robust evidence on organizational risk factors to the health of this group of professionals and to assess the actions and measures which could be taken to mitigate these risks.

Conclusions
In short, most of the research conducted on risk factors for the work organization of aircrew was carried out in the United States and the European Union and had moderate or low-quality methodology and evidence. The findings are homogeneous and allow the most prevalent organizational risk factors for the health of aircrew to be determined, namely, high work demand, long hours, and night work. As a result of these factors, the most prevalent health problems in both professional categories were those related to sleep, mental disorders, musculoskeletal disorders, and fatigue.
Our findings have interesting implications for gaining a better understanding of the main risk factors of aircrew health since this job is significant for society and guides future discussion about the regulation of the aircrew profession. The present review makes it clear that further studies with this professional category need to be carried out, especially longitudinal studies, to understand the causal relationships between occupational risk factors to health.